This invention relates generally to intake and exhaust systems and more particularly to the effective use of shock wave phenomenon to improve the efficiency of an internal combustion engine or a like fluid pumping device having valved intake and exhaust functions.
In, for example, a four-stroke engine, on the first stroke the piston sucks in the fuel and air mixture, on the second stroke the piston compresses the fuel and air mixture, on the third stroke mixture is ignited near the top of the piston travel, causing the piston to be forced downward thereby supplying useful power, and the forth stroke wherein the piston forces the exhaust gases from the cylinder and then the four strokes are sequentially repeated. When the intake valve first opens under normal conditions a negative shock wave is created which draws the air and fuel into the cylinder. When the exhaust valve normally opens a positive shock wave immediately travels through the gases at a speed of 18,500 inches per second (the speed of sound at sea level). When the exhaust shock wave reaches the end of the exhaust system or pipe, a negative shock wave is created and that negative shock wave travels back through the outcoming gases at the same speed as the positive wave. Ideally, this negative shock wave which creates a slight vacuum will reach the exhaust valve prior to its normal closing sequence so that the slight vacuum is present to aid in extracting combustion products from the cylinder.
To utilize the shock wave phenomenon, tuned exhaust systems have been designed. Tuned exhaust systems are length sensitive, that is for a certain engine r.p.m. desired, a specific exhaust system length is required. This length (1) is determined from the following equation: ##EQU1##
Applying the equation to a given engine with a desired operating speed of 6,000 r.p.m., and with cam timing which holds the exhaust valve open for 298.degree. of engine rotation (the exhaust valve opens at 78.degree. before the piston reaching bottom dead center (BBDC) the following tuned exhaust system length for that r.p.m. results: ##EQU2##
As can be seen, if the exhaust length, in the example, is 38.2 inches in length the negative shock wave will normally reach the exhaust port prior to normal exhaust valve closure and engine efficiency will be increased; however, most engines operate through a wide range of r.p.ms, therefore, the engine will have decreased efficiency below or above the ideal engine speed of 6,000 r.p.m.
A number of years ago an attempt was made by Ed Iskenderian Racing Cams of Inglewood, Calif. to add a third lobe to a conventional two lobe cam for use in a super charged racing engine to purge cool the engine by opening the intake valve at 2.5.degree. after bottom dead center (abdc) during the exhaust stroke (after initial blow down when peak pressure is reduced) and holding the valve open for approximately 55.degree. whereby supercharged air from the intake track (at a higher pressure than the exhaust gases) forced the hot exhaust gases from the engine. This attempt to lower engine temperature to prevent pre-ignition was abandoned during dyno testing do to the device being inoperative for the purpose for which it was intended.
It has been found in intake systems that engine volumetric efficiency and power can be increased by taking advantage of the natural dynamic effects which occur during the intake cycle. When the intake valve closes a sonic pressure wave bounces back out the intake track, and then in again toward the valve. By making the intake track the proper length, the returning pressure wave can be timed to arrive at top dead center of the next intake cycle, shoving extra air in and keeping exhaust gases out of the intake port. To use this pulse, the intake port must be the correct length. The pulse will help only through a narrow range of rpm. Above or below a certain range the pulse will actually decrease power so proper synchronization is essential. There are actually several pulses which can be used, corresponding to the 2nd, 3rd and 4th time the sonic wave harmonic arrives at the valve. The 2nd harmonic is the most efficient, the others being weaker and shorter. To obtain the inlet system length, the desired operating rpm (peak) is established, for example; 8,000, and that number is divided into an established length formula number which is 132,000/rpm for the 2nd harmonic, 97,000/rpm for the 3rd harmonic, 74,000/rpm for the 4th harmonic etc. . . This would result in an intake track length of 16.5 inches for the 2nd harmonic, 12.125 inches, and for the 4th harmonic 9.25 inches. In addition to obtain the desired benefit from this negative shock wave phenomenon, it is also necessary that the intake valve open to a lift height of at least 0.02 times the valve diameter by 15.degree. before top dead center (bfdc) while openings of 20.degree. to 40.degree. bfdc are usually preferable and the intake flow rating must be 0.3 or greater for significant benefits.
There is a continuing need to improve the removal of gases of combustion and improved the fuel efficiency of internal combustion engines and the like.